1. Field of the Invention.
This invention relates in general to automatic levels and has specific reference to an automatic level of the type comprising a case, a base supporting said case, means permitting a coarse adjustment on the horizontally of said case, an objective and an eyepiece mounted in said case so that their optical axes be parallel to each other and horizontal when the case is in a horizontal position, a fixed reticule disposed in the object focal plane of the eyepiece, an optical compensating system suspended from said case by means of flexion bars and disposed on the path of the light rays between said objective and said reticule, an image erecting system disposed between said objective and said reticule, and an image focusing device.
2. Description of the Prior Art.
It is known that in levels it is essential that the line of sight be exactly horizontal, or in other words that the horizontal light ray passing through the center of the lens also passes through the center of the reticule, even if the level case is not strictly horizontal. The adjustment means generally provided on the level base and an air bubble level carried by the level case are not sufficient for achieving this result, for they afford only a coarse or rough adjustment of the level horizontality.
Therefore, means had to be provided for automatically compensating the defective horizontality of the level case, so that the line of sight be perfectly horizontal after setting the level in position and a rough adjustment of its horizontality.
Many compensating systems have already been proposed for automatic levels. As a rule, these compensating systems utilize mirrors or prisms with a pendular suspension or with a flexion bar suspension, in order to provide a reflecting surface forming a constant angle with the vertical. Compensating systems of this type have been disclosed notably in U.S. Pat. No. 3,628,874 and in French Pat. Nos. 1,386,114; 2,162,534 and 2,189,709. Other automatic levels utilize a liquid blade having a reflecting surface remaining constantly horizontal for compensating the lack of horizontality. Typical automatic levels of this kind are illustrated and described in the British Pat. No. 859,617 and in the French Pat. No. 2,221,788.
On the other hand, the objective of a level forms an image of the sighted object which is inverted both from right to left and from top to bottom. Therefore, some known automatic levels incorporate an optical image erecting system. This system may consist of means differring completely from the compensating system, as disclosed for example in the French Pat. No. 2,036,372 providing for this purpose a prism consisting of one portion of an Abbe prism, which accomplishes a complete erection of the image. In the other case, the image erecting system is sometimes combined with the compensating system, as illustrated for example in the French Pat. Nos. 1,386,114 and 2,162,534. However, in these two last-mentioned documents the image is not erected completely but only in the top to bottom direction.
Finally, in known automatic levels, an image focusing device is also contemplated which consists in general of an optical system separate from the compensating system and also from the image erecting system, this focusing device consisting as a rule of a vehicle incorporating a divergent optical system.
From the foregoing it is clear that hitherto known automatic levels comprises a relatively great number of separate optical systems for compensating the insufficient horizontality, partially or totally erecting the image, and focusing the image. Obviously, this combination of means increases considerably the cost of the apparatus as well as its over-all dimensions.